Method and apparatus for improving radio location accuracy with measurements

ABSTRACT

A method and apparatus to utilize a set of measurements (either partial or compete) to improve the accuracy of an initial position estimate for a wireless terminal. The initial position estimate for the terminal is first obtained (e.g., based on a cell-ID or an enhanced cell-ID solution). Measurements are obtained for the terminal. The initial position estimate is then updated with the measurements to obtain a revised position estimate for the terminal. The updating may be performed by (1) deriving a measurement vector based on the initial position estimate and the measurements, (2) forming an observation matrix for the measurements, (3) determining a matrix of weights, (4) deriving a correction vector based on the measurement vector, the observation matrix, and the weight matrix, and (5) updating the initial position estimate with the correction vector.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/700,633, filed Feb. 4, 2010, entitled “Method And Apparatus ForImproving Radio Location Accuracy With Measurements,” which is acontinuation of and claims priority to U.S. patent application Ser. No.10/418,799, filed Apr. 17, 2003, entitled “Method And Apparatus ForImproving Radio Location Accuracy With Measurements,” which claims thebenefit of U.S. Provisional Application No. 60,419,680, filed on Oct.17, 2002, and U.S. Provisional Application No. 60/433,055, filed on Dec.13, 2002, all of which are assigned to the assignee hereof and which areexpressly incorporated herein by reference.

BACKGROUND

1. Field

The present invention relates generally to position determination. Morespecifically, the present invention relates to a method and apparatusfor providing a more accurate position estimate for a wireless terminalusing a set of measurements.

2. Background

It is often desirable, and sometimes necessary, to know the position ofa wireless user. For example, the Federal Communications Commission(FCC) has adopted a report and order for enhanced 911 (E9-1-1) wirelessservice that requires the location of a wireless terminal (e.g., acellular phone, modem, computer with wireless capability, personaldigital assistant (PDA), or any other such mobile or portable devicethat has wireless communication capability) to be provided to a PublicSafety Answering Point (PSAP) each time a 911 call is made from theterminal. The FCC mandate requires the location of the terminal, forhandset-based technologies such as Assisted-GPS (A-GPS), to be accurateto within 50 meters for 67% of calls and within 150 meters for 95% ofcalls. In addition to the FCC mandate, service providers may uselocation services (i.e., services that identify the position of wirelessterminals) in various applications to provide value-added features thatmay generate additional revenue.

Various systems may be used to determine the position of a wirelessterminal. One such system is the well-known Global Positioning System(GPS), which is a “constellation” of 24 well-spaced satellites thatorbit the earth. Each GPS satellite transmits a signal encoded withinformation that allows receivers to measure the time of arrival of thereceived signal relative to an arbitrary point in time. This relativetime-of-arrival measurement may then be converted to a “pseudo-range”,which is the sum of the actual range between the satellite and theterminal plus all errors associated with the measurement. Athree-dimensional position of a GPS receiver may be accurately estimated(to within 10 to 100 meters for most GPS receivers) based onpseudo-range measurements to a sufficient number of satellites(typically four) and their locations.

A wireless communication system, such as a cellular communicationsystem, may also be used to determine the position of a wirelessterminal. Similar to the GPS signals, a terminal may receive a“terrestrial” signal from an earth-bound base station and determine thetime of arrival of the received signal. Again, the time-of-arrivalmeasurement may be converted to a pseudo-range. Pseudo-rangemeasurements to a sufficient number of base stations (typically three ormore) may then be used to estimate a two-dimensional position of theterminal.

In a hybrid position determination system, signals from earth-bound basestations may be used in place of, or to supplement, signals from GPSsatellites to determine the position of a wireless terminal. A “hybrid”terminal would include a GPS receiver for receiving GPS signals from thesatellites and a “terrestrial” receiver for receiving terrestrialsignals from the base stations. The signals received from the basestations may be used for timing by the terminal or may be converted topseudo-ranges. The three-dimensional position of the terminal may beestimated based on a sufficient number of measurements for thesatellites and base stations (for CDMA networks it is typically four).

The three different position determination systems described above(namely GPS, wireless, and hybrid) can provide position estimates (or“fixes”) with different levels of accuracy. A position estimate derivedbased on signals from the GPS is the most accurate. However, GPS signalsare received at very low power levels due to the large distances betweenthe satellites and the receivers. Moreover, most conventional GPSreceivers have great difficulty receiving GPS signals inside buildings,under dense foliage, in urban settings in which tall buildings blockmuch of the sky, and so on. A position estimate derived from the hybridsystem is less accurate, and one derived based on signals from thewireless communication system is even less accurate. This is becausepseudo-ranges computed based on signals from the base stations are proneto exhibit larger errors than those computed from GPS signals due totiming and hardware errors in the base stations, timing and hardwareerrors in the terminal, and errors due to the terrestrial propagationpath.

The position of a terminal may be estimated based on any one of thethree systems described above. It is desirable to obtain a positionestimate that is as accurate as possible. Thus, a GPS solution would bederived if a sufficient number of GPS signals are available. If such isnot the case, then a hybrid solution may be derived if one or more GPSsignals plus a sufficient number of terrestrial signals are available.And if no GPS signals are available, then a cellular solution may beobtained if a sufficient number of terrestrial signals are available.

The required number of signals to derive any one of the three solutionsdescribed above may not be available. In such situations, somealternative position determination technique may be used to estimate theterminal's position. One such alternative technique is the cell-IDtechnique, which provides a designated location for a reference (orserving) base station with which the terminal is in communication as theterminal's position estimate. This designated location may be the centerof the base station' coverage area, the location of the base stationantenna, or some other location within the coverage area of the basestation. An enhanced cell-ID solution may combine cell-ID informationfrom a reference base station with cell-ID information from another basestation and/or include a round-trip delay measurements and/or signalstrength measurements from at least one base station which is incommunication with the terminal A cell-ID or enhanced cell-ID solutionmay be provided as a “fall-back” or “safety-net” solution when a moreaccurate solution cannot be independently derived because a sufficientnumber of signals is not available. Unfortunately, since the quality ofthe position estimate provided by the above mentioned alternativetechnique is dependent on the size of the base station's coverage area,it may be quite poor.

There is therefore a need in the art for a method and apparatus toprovide a more accurate position estimate for the terminal usingmeasurements that are available.

SUMMARY

A method and apparatus is described herein to utilize a positionlocation measurements to improve the accuracy of an initial positionestimate for a wireless terminal. These measurements may be eitherpartial set of measurements or a “complete” set of measurements. Apartial measurement set includes measurements that are available but notin sufficient number to produce an independent position fix for theterminal with a predetermined quality of service (i.e., predeterminedaccuracy). However, instead of discarding these measurements, as isnormally done, they are used to derive a revised position estimate forthe terminal having improved accuracy over the initial positionestimate. In another method and apparatus, an initial position estimateis improved by using a complete set of measurements. A complete set ofmeasurements is a set of measurements from which it is possible toderive a position location solution with a sufficiently high quality ofservice, but which can nonetheless be improved by the method andapparatus. This method and apparatus is essentially the same whether acomplete set or a partial set of measurements is used. Accordingly, forease of discussion, the disclosed method and apparatus is described inthe context of the partial set of measurements only.

In one method for determining a position estimate for the wirelessterminal, the initial position estimate for the terminal is firstobtained based on a cell-ID or an enhanced cell-ID solution or otherposition location estimation schemes. A partial set of measurements isalso obtained for the terminal from one or more position determinationsystems. The partial set may include measurements from satellites,wireless base stations and/or access points or a combination ofsatellite and terrestrial measurements. The initial position estimate isthen updated with the partial set of measurements to obtain the revisedposition estimate for the terminal

The updating may be performed by first deriving a measurement vectorbased on the initial position estimate and the partial set ofmeasurements. The measurement vector typically includes pseudo-rangeresiduals for the transmitters whose measurements are in the partialset. Each pseudo-range residual is the difference between (1) a“measured” pseudo-range from the terminal's position to the transmitter(derived based on the measurement) and (2) a “computed” pseudo-rangefrom the initial position estimate to the transmitter. An observationmatrix is also formed for the partial set of measurements. A matrix ofweights to use in the combining of the initial position estimate and thepartial set of measurements may also be determined. A correction vectoris then derived based on the measurement vector, the observation matrix,and the weight matrix. The initial position estimate is then updatedwith the correction vector, which includes changes to the initialposition estimate.

Various aspects and embodiments of the method and apparatus aredescribed in further detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

The features, nature, and advantages of the present invention willbecome more apparent from the detailed description set forth below whentaken in conjunction with the drawings in which like referencecharacters identify correspondingly throughout and wherein:

FIG. 1 illustrates a system comprised of a number of positiondetermination systems;

FIG. 2 shows a process for providing a more accurate position estimatefor a wireless terminal using a partial set of measurements;

FIGS. 3A through 3C illustrate three example operating scenarios wherethe disclosed method and apparatus can provide the more accurateposition estimate;

FIGS. 4A through 4E graphically illustrate the process of combining theinitial position estimate with satellite and/or cellular measurements;

FIG. 5 shows a specific embodiment of the process shown in FIG. 2;

FIG. 6 shows a process for combining state domain information withmeasurement domain information to provide the more accurate positionestimate; and

FIG. 7 is a block diagram of an embodiment of a receiver unit, which maybe a component of the wireless terminal.

DETAILED DESCRIPTION

FIG. 1 is diagram illustrating a system 100 comprised of a number ofposition determination systems. One such position determination systemis a satellite positioning system (SPS), which may be the well-knownGlobal Positioning System (GPS). Another such position determinationsystem is a cellular communication system, which may be a Code DivisionMultiple Access (CDMA) communication system, a Global System for Mobile(GSM) communication system, or some other wireless system. In general,system 100 may include any number of position determination systems thatmay be of any type (e.g., a Bluetooth, Wireless Fidelity (Wi-Fi),Ultra-Wide Bandwidth (UWB) or any other system capable of providinglocation related information). If the system is designed to providelocalized signal coverage, then this system may be referred to as aLocal Area Positioning System (LAPS).

As shown in FIG. 1, a terminal 110 may receive signals transmitted froma number of transmitters (or transceivers), each of which may be a basestation 120 of the cellular communication system or a satellite 130 ofthe SPS. The terminal 110 may be a cellular phone, modem, computer withwireless capability, personal digital assistant (PDA), or any other suchmobile or portable device that has wireless communication capability. Ingeneral, any type of transmitter placed at the locations that are knownor can be ascertained may be used to determine the terminal's position.For example, terminal 110 may receive signal from an access point in aBluetooth system. As used herein, a base station may be any earth-boundtransmitter or transceiver that transmits and/or receives a signal thatmay be used for position determination.

Terminal 110 may be any device capable of receiving and processingsignals from the position determination systems to obtain timing,ranging, and/or location information. It should be noted that timing andranging do not need to be tied together. For instance, simply receivinga signal from a short-range system such as a Bluetooth system mayprovide sufficient information to radio-locate a terminal. Terminal 110may be a cellular telephone, a fixed terminal, an electronics unit(e.g., a computer system, a personal digital assistant (PDA), and so on)with a wireless modem, a receiver unit capable of receiving signals fromsatellites and/or base stations, and so on. In another example, terminal110 may be any device capable of transmitting signals to the positiondetermination systems so that these position determination systems mayobtain timing, ranging, and/or location information.

The position of a wireless terminal may be determined based on signalsfrom one or multiple position determination systems. For example, ifsystem 100 includes the SPS and cellular communication system, then theposition of the terminal may be estimated based on signals from (1) theSPS alone, (2) the cellular communication system alone, or (3) both theSPS and cellular communication system. Techniques for determining theposition of the terminal based solely on measurements for base stationsin the cellular communication system are known as Advanced Forward LinkTrilateration (A-FLT), Uplink Time of Arrival (U-TOA) or Uplink TimeDifference of Arrival (U-TDOA), Enhanced Observed Time Difference(E-OTD), and Observed Time Difference of Arrival (OTDOA).

Each position determination system can provide position estimates (orfixes) with a certain level of accuracy and may also be available forcertain operating environments. If system 100 includes the SPS andcellular communication system, then the accuracy and availability forthese systems may be briefly summarized (in typical descending order ofaccuracy) as shown in Table 1.

TABLE 1 Measure- ment Solution Type Type Description SPS Hand- Solutionbased solely on SPS. Highest accuracy. set- May not be available forcertain environments based (e.g., deep indoors). SPS + Hybrid Hybridsolution based on a combination of SPS A-FLT and cellular communicationsystems. Intermediate accuracy. Improved indoor availability. LAPS WLAN-Solution based solely on the local-area based communication system.Accuracy dependent on the system's maximum range characteristics. Verygood indoor availability. A-FLT Net- Solution based solely on thecellular communication work- system. Reduced accuracy. Commonlyavailable in based urban area and may be available where GPS is notavailable (e.g., deep indoors). Enhanced Cell- Solution based solely onthe cellular communication Cell-ID based system. Low accuracy. Generallydepends on the cell sector size and the accuracy of round trip delay orsimilar measurement. May include other cellular measurements such asobservations of more than one transmitter, and signal strength. Cell-IDCell- Solution based solely on the cellular communication based system.Lowest accuracy. Provides only the identity of the cell where theterminal is located. Therefore, accuracy is dependent on the size of thecell.

An “SPS-based” solution has the highest accuracy in Table 1. However, asufficient number of SPS satellites (typically four) may not beavailable in certain operating environments (e.g., indoors) to computethis solution. A “hybrid” solution has the next highest accuracy butrequires signals from one or more SPS satellites plus a sufficientnumber of base stations. Again, the required number of signals(typically four) may not be available for certain operatingenvironments. A “network-based” solution such as A-FLT may be obtainedbased on measurements for a sufficient number of base stations (three ormore). If the required number of base stations is not available, then a“cell-based” cell-ID or enhanced cell-ID solution may be obtained basedon a measurement for a single base station. This base station istypically the one that is in communication with the terminal, and isoften referred to as the “reference” base station. In another example,the enhanced cell-ID solution may include information from multiple basestations or cells such as the cell coverage area descriptions, theobservations from multiple transmitters and signal characteristics suchas signal strength, signal interference, etc.

Techniques for deriving a hybrid solution are described in detail inU.S. Pat. No. 5,999,124, entitled “Satellite Positioning SystemAugmentation with Wireless Communication Signals,” issued Dec. 7, 1999,which is incorporated herein by reference.

Conventionally, one of the solutions shown in Table 1 is providedwhenever a position estimate is needed for the terminal. The mostaccurate solution is derived if the required number of measurements(i.e., a complete set of measurements) for the solution is available. Iffewer than the required number of measurements is available, then afall-back or safety-net solution such as a cell-ID or enhanced cell-IDsolution may be provided.

A method and apparatus is described herein to utilize a partial set ofmeasurements obtained from one or more position determination systems toimprove the accuracy of a coarse initial position estimate. The initialposition estimate may be provided, for example, by a cell-ID, enhancedcell-ID or LAPS solution. It will be understood by those skilled in theart that several other ways are known for determining an initialposition estimate, such as by use of dead reckoning, an estimatedirectly input by the user, etc.

The partial set may include SPS and/or cellular measurements. Thispartial set is defined by the fact that it does not include a sufficientnumber of measurements needed to derive an independent position estimatefor the terminal with a predetermined quality of service. It will beunderstood by those skilled in the art that the predetermined quality ofservice shall be determined based upon the particular application forwhich the position location determination will be used. For example, thequality of service required for providing information about what pointsof interest (e.g., automatic teller machines (ATMs), restaurants, storesof a particular type, etc.) are nearby might be relatively low(inaccurate). In contrast, the predetermined quality of service wouldneed to be relatively high (accurate) for an application such asnavigating through a labyrinth of narrow streets separated by relativelysmall distances. Even higher quality might be required to provideinformation about a particular store or restaurant in which you happento be located. For example, in one application, the user of a terminalmight be interested in downloading the menu of the restaurant he isabout to enter on a street that has several competing restaurants invery close proximity to one another (i.e., next door to each other). Inorder to distinguish one from the other, the quality of service wouldneed to be relatively high.

However, instead of discarding measurements that are insufficient toachieve the predetermined quality of service, as is conventionally done,the presently disclosed method and apparatus uses these measurements toderive a revised position estimate having improved accuracy over theinitial position estimate. One exception may be a LAPS solution. Ifeither the LAPS maximum signal range or the distance from the LAPStransmitter is smaller then an initial position error estimate, then theinitial position estimate may be updated (or replaced) by the LAPSsolution, which may have been derived from a single LAPS measurement.This LAPS measurement may be a range measurement, a signalcharacteristic, a simple indicator of signal reception, or it may bebased on the description of the LAPS coverage area.

In another method and apparatus, an initial position estimate isimproved by using a complete set of measurements. A complete set ofmeasurements is a set of measurements from which it is possible toderive a position location solution with a sufficiently high quality ofservice, but which can nonetheless be improved by the method andapparatus. The presently disclosed method and apparatus is essentiallythe same whether a complete set or a partial set of measurements isused. Accordingly, for ease of discussion, the disclosed method andapparatus is described in the context of the partial set of measurementsonly.

FIG. 2 is a flow diagram of an embodiment of a process 200 for providinga more accurate position estimate for a wireless terminal using apartial set of measurements. The process starts off by obtaining aninitial position estimate for the terminal (step 212). This initialposition estimate may be derived from one or more position determinationsystems. Furthermore, the initial position estimate can represent themost accurate solution that can be obtained using any positiondetermination technique available. For example, the initial positionestimate may be provided by a cell-ID solution, an enhanced cell-IDsolution, or some other solution.

A partial set of measurements is also obtained from one or more positiondetermination systems (step 214). This partial set does not include asufficient number of measurements to derive an independent positionestimate for the terminal with a predetermined quality of service.However, if the required number of measurements were available, then theindependent position estimate could have been obtained for the terminal,and this position estimate would typically have higher accuracy than theinitial position estimate. The partial set may include measurements fromonly the SPS, measurements from only the cellular communication system,or measurements from both the SPS and wireless communication system orfrom any number of other position determination systems.

The initial position estimate is then updated with the partial set ofmeasurements to obtain a revised position estimate for the terminal(step 216). This revised position estimate has higher accuracy than theinitial position estimate. The amount of improvement in accuracy isdependent on various factors such as (1) the accuracy (or inaccuracy) ofthe initial position estimate, (2) the number and type of measurementsavailable for updating, geometry (i.e., the relative locations of thetransmitters from which signals are received to and so on. The updatingis described below.

To more clearly describe the method and apparatus, the derivation tocompute a position estimate for the terminal based on a complete set ofmeasurements is first described. In the following description, ageodetic coordinate system is used and a three-dimensional (3-D)position can be defined by three values for latitude (north), longitude(east), and altitude (up).

For a terminal located at a given 3-D coordinate, its exact position canbe determined based on actual (or “true”) ranges to three transmittersat known locations. However, the true range to each transmitter normallycannot be determined because of clock and other measurement errors.Instead, a “pseudo-range” can be determined, which includes the truerange plus an offset due to clock and other measurement errors. A fourthmeasurement would then be needed to remove the common offset in all ofthe measurements.

A basic equation relating the terminal's position, the i-thtransmitter's location, and the pseudo-range PR_(i) from the terminal'sposition to the i-th transmitter location may be expressed as:

PR_(i)=√{square root over((Lat−Lat_(i))²+(Long−Long_(i))²+(Alt−Alt_(i))²)}{square root over((Lat−Lat_(i))²+(Long−Long_(i))²+(Alt−Alt_(i))²)}{square root over((Lat−Lat_(i))²+(Long−Long_(i))²+(Alt−Alt_(i))²)}+T,  Eq (1)

where

-   -   Lat, Long, and Alt represent the 3-D planar spatial coordinates        of the terminal's actual position;    -   Lat_(i), Long_(i), and Alt_(i) represent the coordinates of the        i-th transmitter location; and    -   T represents the temporal coordinate.        A set of four basic equations may be obtained as shown in        equation (1) for four different transmitters, i.e., for i={1, 2,        3, 4}.

The basic equations may be linearized by employing incrementalrelationships, as follows:

Long=Long_(init) +Δe,

Lat=Lat_(init) +Δn,

Alt=Alt_(init) +Δu,

T=T _(init) +ΔT, and

PR_(i)=PR_(init,i)+ΔPR_(i), for i={1, 2, 3, 4},  Eq (2)

where

-   -   Lat_(init), Long_(init), Alt_(init), and T_(init) are the        initial values (a priori best estimate) of Lat, Long, Alt, and        T, respectively;    -   Δe, Δn, Δu, and ΔT represent the corrections to the initial        values Lat_(init), Long_(init), Alt_(init), and T_(init),        respectively;    -   PR_(init,i) represents the pseudo-range measurement from the        initial position estimate to the i-th transmitter (i.e., a        “computed” pseudo-range);    -   PR_(i) represents the pseudo-range measurement from the        terminal's position to the i-th transmitter (i.e., a “measured”        pseudo-range); and    -   ΔPR_(i) represents the difference between the computed and        measured pseudo-ranges (which is also referred to as the        “pseudo-range residual”).

In equation set (2), Lat_(init), Long_(init), and Alt_(init) representthe terminal's initial 3-D position estimate, and Lat, Long, and Altrepresent the terminal's actual 3-D position (or an a posteriori bestestimate). The initial position estimate is the best estimate currentlyavailable for the terminal.

The pseudo-range measurement PR_(init,i) is a computed value for thepseudo-range between the initial position estimate (Lat_(init),Long_(init), and Alt_(init)) and the known location of the i-thtransmitter (Lat_(i), Long_(i), and Alt_(i)). This pseudo-rangemeasurement may be expressed as:

PR_(init,i)=√{square root over((Lat_(init)−Lat_(i))²+(Long_(init)−Long_(i))²+(Alt_(init)−Alt_(i))²)}{squareroot over((Lat_(init)−Lat_(i))²+(Long_(init)−Long_(i))²+(Alt_(init)−Alt_(i))²)}{squareroot over((Lat_(init)−Lat_(i))²+(Long_(init)−Long_(i))²+(Alt_(init)−Alt_(i))²)}.  Eq(3)

The pseudo-range measurement PR_(i) is considered a “measured” valuebecause it is derived based on the signal received by the terminal fromthe i-th transmitter. In particular, if the time the signal istransmitted from the i-th transmitter is known (e.g., if the signal istime-stamped or timing information is encoded in the signal), then thetime it takes the signal to travel to the terminal can be determined byobserving the time the signal is received at the terminal (based on theterminal's internal clock). However, the amount of time betweentransmission and reception typically cannot be determined exactlybecause of offsets between the clocks at the transmitter and terminaland other measurement errors. Thus, a pseudo-range is derived based onthe difference between a reference time and the time that the signal isreceived. In another example, a signal characteristic such as a signalstrength or a combination of signal characteristics can be used toderive a pseudo-range measurement. The derivation of a pseudo-range froma signal received from an SPS satellite is known in the art and notdescribed in detail herein.

The pseudo-range residual ΔPR_(i) for the i-th transmitter may beexpressed as:

ΔPR_(i)=PR_(i)−PR_(init,i).  Eq (4)

Substituting the incremental expressions in equation set (2) into thebasic equation (1) and ignoring second-order error terms, the followingcan be obtained:

$\begin{matrix}{{{\Delta \; {PR}_{i}} = {{\frac{\partial{PR}_{i}}{\partial e}\Delta \; e} + {\frac{\partial{PR}_{i}}{\partial n}\Delta \; n} + {\frac{\partial{PR}_{i}}{\partial u}\Delta \; u} + {\Delta \; T}}},{{{for}\mspace{14mu} i} = {\left\{ {1,2,3,4} \right\}.}}} & {{Eq}\mspace{14mu} (5)}\end{matrix}$

The four linearized equations shown by equation (5) may be moreconveniently expressed in a matrix form, as follows:

$\begin{matrix}{{\begin{bmatrix}{\Delta \; {PR}_{1}} \\{\Delta \; {PR}_{2}} \\{\Delta \; {PR}_{3}} \\{\Delta \; {PR}_{4}}\end{bmatrix} = {\begin{bmatrix}\frac{\partial}{\partial e} & \frac{\partial}{\partial n} & \frac{\partial}{\partial u} & 1 \\\frac{\partial}{\partial e} & \frac{\partial}{\partial n} & \frac{\partial}{\partial u} & 1 \\\frac{\partial}{\partial e} & \frac{\partial}{\partial n} & \frac{\partial}{\partial u} & 1 \\\frac{\partial}{\partial e} & \frac{\partial}{\partial n} & \frac{\partial}{\partial u} & 1\end{bmatrix}*\begin{bmatrix}{\Delta \; e} \\{\Delta \; n} \\{\Delta \; u} \\{\Delta \; T}\end{bmatrix}}},} & {{Eq}\mspace{14mu} (6)}\end{matrix}$

where

$\frac{\partial}{\partial x}$

is the direction cosine of the angle between the pseudo-range to thei-th transmitter and a vector in the x direction, where x can be east,north, or up. Equation (6) may be used to determine or update theterminal's position, provided that a complete and independent set ofpseudo-range measurements for four transmitters is available.

FIG. 3A is a diagram illustrating an example operating scenario wherethe disclosed method and apparatus may be used to provide a moreaccurate position estimate. In FIG. 3A, terminal 110 receives a signalfrom base station 120 x and signals from two SPS satellites 130 x and130 y. These three signals may not be sufficient to derive a 3-D hybridposition fix. A cell-ID solution may then be derived using basicknowledge of base station 120 x, which is in communication with terminal110. If base station 120 x is designed to provide coverage for ageographic area approximated by a circle 310, then the position ofterminal 110 may be estimated as the location of the base station orsome other designated location within the coverage area.

To increase system capacity, the coverage area of each base station maybe partitioned into a number of sectors (e.g., three sectors). Eachsector is then served by a corresponding base transceiver subsystem(BTS). For a coverage area that has been sectorized (commonly referredto as a sectorized cell), the base station serving that coverage areawould then include all BTSs serving the sectors of the coverage area. Anenhanced cell-ID solution may then be obtained with additionalinformation identifying the specific BTS sector with which the terminalis in communication. In such a case, the uncertainty in the terminal'sposition can be reduced to a pie-shaped area, which is labeled as sectorA in FIG. 3A. The position of the terminal may then be estimated as thecenter of the sector covered by this BTS (point 312) or some otherdesignated location.

Additional information may also be available, such as the strength ofthe signal received from the BTS, the round trip delay (RTD) between theterminal and the BTS, the time advance (TA) of the received signal (forGSM), the round trip time (RTT) between the terminal and BTS (forW-CDMA), and so on. If such additional information is available, thenthe position estimate of the terminal may be adjusted accordingly.

As illustrated above, the cell-ID or enhanced cell-ID technique canprovide a coarse position estimate for the terminal. This would thenrepresent the 2-D a priori best estimate (i.e., the initial positionestimate) for the terminal. The initial position estimate for theterminal may be given as (Lat_(init) and Long_(init)). A revisedposition estimate having improved accuracy may then be obtained for theterminal using two pseudo-range measurements for the two SPS satellites130 x and 130 y.

The linearized equations for the terminal with two pseudo-rangemeasurements for two satellites may be expressed as:

$\begin{matrix}{{\begin{bmatrix}{\Delta \; {PR}_{1}} \\{\Delta \; {PR}_{2}} \\{\Delta \; H} \\{\Delta \; {CB}}\end{bmatrix} = {\begin{bmatrix}\frac{\partial}{\partial e} & \frac{\partial}{\partial n} & \frac{\partial}{\partial u} & 1 \\\frac{\partial}{\partial e} & \frac{\partial}{\partial n} & \frac{\partial}{\partial u} & 1 \\0 & 0 & 1 & 0 \\0 & 0 & 0 & 1\end{bmatrix}*\begin{bmatrix}{\Delta \; e} \\{\Delta \; n} \\{\Delta \; u} \\{\Delta \; T}\end{bmatrix}}},} & {{Eq}\mspace{14mu} (7)}\end{matrix}$

where

-   -   ΔH is the altitude residual, which represents the difference        between the current estimate of the terminal's altitude and the        actual altitude; and    -   ΔCB represents the difference between the current reference time        estimate and the “true” reference time.

In equation (7), the pseudo-range residual ΔPR_(i) for each of the twoSPS satellites may be determined based on the computed pseudo-rangePR_(init,i), and the measured pseudo-range PR_(i) for the satellite, asshown in equation (4). The pseudo-range PR_(init,i), may be computed asthe distance between the terminal's initial position estimate(Lat_(init), Long_(init), and Alt_(init)) and the i-th satellite'slocation (Lat_(i), Long_(i), and Alt_(i)), where the terminal'saltitude, Alt_(init), may be estimated to be equal to the altitude forthe serving BTS or some other altitude. Given some additionalinformation about the reference time, ΔCB can be used to account for thedifference between the current reference time estimate and the “true”reference time. In one example, the propagation time between the servingBTS and the terminal can be measured and used to provide the informationabout the reference time delay. The pseudo-range PR_(i) is derived basedon the signal received from the i-th satellite and is a measurement ofthe range from the i-th satellite to the terminal's actual (“true”)location.

Equation (7) may also be expressed in a more compact form as follows:

r=Hx,  Eq (8)

where

-   -   r is a vector with four elements for the pseudo-range residuals        (i.e., the “measurement” vector);    -   x is a vector with four elements for the user position and time        corrections (i.e., the “correction” vector); and    -   H is the 4×4 “observation” matrix.

The correction vector x may then be determined as:

x=H ⁻¹ r.  Eq (9)

Equation (9) provides an unweighted solution for the correction vectorx. This equation gives equal weights to the information related to theinitial position estimate (e.g., obtained from the cell-ID or some othertechnique) and the ranging information for the SPS satellites. To bettercombine the two pieces of information, the initial position estimate andpseudo-range measurements may be assigned appropriate weights.

A covariance matrix V, which is also known as a measurement noisematrix, may be determined for the linearized equations shown in equationset (7) and may be expressed as:

$\begin{matrix}{{\underset{\_}{V} = \begin{bmatrix}V_{11} & 0 & 0 & 0 \\0 & V_{22} & 0 & 0 \\0 & 0 & V_{h} & 0 \\0 & 0 & 0 & V_{cb}\end{bmatrix}},} & {{Eq}\mspace{14mu} (10)}\end{matrix}$

where

-   -   V₁₁ is the variance of the error for the pseudo-range        measurement PR₁ for the first satellite;    -   V₂₂ is the variance of the error for the pseudo-range        measurement PR₂ for the second satellite;    -   V_(h) is the variance of the error for the height measurement;        and    -   V_(cb) is the variance of the error associated with the        reference time.        The elements V₁₁ and V₂₂ may be expressed as V₁₁=σ_(pr1) ² and        V₂₂=σ_(pr2) ², where σ_(pr1) and σ_(pr2) are the standard        deviations of the errors for the pseudo-range measurements PR₁        and PR₂, respectively. A weight matrix W may be defined as an        inverse of the covariance matrix V (i.e., W=V⁻¹). The non-zero        elements of W determine the weighting for the pseudo-range        measurements and the information related to the initial position        estimate in the derivation of the revised position estimate. The        elements of W are inversely related to the expected values of        the squares or cross-products of the errors in the measurements.        Thus, a small error for any quantity (e.g., PR_(i)) means a more        reliable observation and corresponds to a large corresponding        value for W. This would then result in that quantity being given        higher weight in the combining of the initial position estimate        with the pseudo-range measurements.

The pseudo-range PR_(i) to the i-th satellite may be defined as:

PR_(i) =R _(i) +CB+SV _(i) +Tr _(i) +I _(i) +M _(i)+η_(i),  Eq (11)

where

-   -   R_(i) is the true or actual range from the terminal position to        the i-th satellite;    -   CB represents the error due to the reference time;    -   SV_(i) represents all errors associated with the i-th satellite;    -   Tr_(i) represents errors due to the SPS signal passing through        the troposphere;    -   I_(i) represents errors due to the SPS signal passing through        the ionosphere;    -   M_(i) represents the error associated with the signal        propagation environment, which includes multipath; and    -   η_(i) represents the error associated with receiver measurement        noise (or thermal noise).

The error estimate V_(ii) would then include all the errors in thepseudo-range measurement for the i-th satellite. Equation (10) assumesthat the pseudo-range measurements are mutually independent. Thederivation of the measurement noise matrix V is known in the art and isnot described in detail herein.

A weighted solution for the correction vector x may then be expressedas:

x=(H ^(T) WH)⁻¹ H ^(T) Wr,  Eq (12)

where H^(T) represents the transpose of H.

Equation (9) or (12) may be used to obtain the correction vector x. Thisvector would include two non-zero terms for Δe and Δn. The revised2D-position estimate for the terminal may then be computed as:

Long_(rev)=Long_(init) +Δe, and

Lat_(rev)=Lat_(init) +Δn.  Eq (13)

The process of combining the initial position estimate with the SPSand/or other measurements is described in further detail below withreference to FIGS. 4A through 4D.

FIG. 3B is a diagram illustrating another example operating scenariowhere the disclosed method and apparatus may be used to provide a moreaccurate position estimate. In FIG. 3B, terminal 110 receives twosignals from base stations 120 x and 120 y. These two signals are notsufficient to derive a network-based (e.g., A-FLT) position fix. Acell-ID or enhanced cell-ID solution may be derived based on thelocation of the base station designated as the terminal's serving basestation, similar to that described above for FIG. 3A. The initialposition estimate for the terminal may be given as Lat_(init) andLong_(init).

Similar to SPS satellites, the pseudo-range to each base station may beestimated based on the signal received from the base station. For a CDMAsystem, each base station is assigned a pseudo-random noise (PN)sequence with a specific offset (or starting time). This PN sequence isused to spectrally spread data prior to transmission from the basestation. Each base station also transmits a pilot, which is simply asequence of all ones (or all zeros) that is spread with the assigned PNsequence. The signal transmitted by the base station is received at theterminal, and the arrival time of the signal may be determined based onthe phase of the PN sequence used for spreading. Since the pilot istypically processed to obtain this PN phase information, thismeasurement at the terminal is also known as a pilot phase measurement.The pilot phase measurement is used to estimate the amount of time ittakes the signal to travel from the base station to the terminal Thistravel time may be converted to a pseudo-range similar to that performedfor the SPS satellite. A pseudo-range measurement derived from aterrestrial signal (e.g., a pilot phase measurement) is denoted as PP todifferentiate it from a pseudo-range measurement derived from an SPSsignal.

The linearized equations for the terminal with two pseudo-rangemeasurements for two base stations may be expressed as:

$\begin{matrix}{\begin{bmatrix}{\Delta \; {PP}_{1}} \\{\Delta \; {PP}_{2}} \\0 \\0\end{bmatrix} = {\begin{bmatrix}\frac{\partial}{\partial e} & \frac{\partial}{\partial n} & 0 & 1 \\\frac{\partial}{\partial e} & \frac{\partial}{\partial n} & 0 & 1 \\0 & 0 & 1 & 0 \\0 & 0 & 0 & 1\end{bmatrix}*{\begin{bmatrix}{\Delta \; e} \\{\Delta \; n} \\{\Delta \; u} \\{\Delta \; T}\end{bmatrix}.}}} & {{Eq}\mspace{14mu} (14)}\end{matrix}$

As shown in equation (14), the terminal and base stations are assumed tobe on the same altitude plane and there are no

$\frac{\partial}{\partial u}$

terms in the observation matrix. However, depending on the relativegeometry (e.g., the BTS may be on a hill and the terminal may be in avalley), there may be observability in the vertical direction for a PPmeasurement. In this case, it would be appropriate to include partialderivative terms with respect to “up” (i.e.,

$\frac{\partial}{\partial u}$

terms) in the first two rows of the observation matrix. Equation (14)shows that the pseudo-range residual ΔPP computation for a terrestrialsignal is similar to the pseudo-range residual ΔPR computation for anSPS signal, which is shown in equation (7). An alternative method forcomputing position estimate is an algebraic solution withoutlinearization.

The correction vector x may then be solved for by using equation (9) or(12) and would include two non-zero terms for Δe and Δn. The revisedposition estimate for the terminal (Lat_(rev) and Long_(rev)) may thenbe computed as shown in equation (13).

FIG. 3C is a diagram illustrating yet another example operating scenariowhere the disclosed method and apparatus may be used to provide a moreaccurate position estimate. In FIG. 3C, terminal 110 receives a signalfrom base station 120 x and a signal from SPS satellite 130 x. These twosignals are not sufficient to derive a hybrid position fix. A cell-ID orenhanced cell-ID solution may be derived based on the location of basestation 120 x, as described above for FIG. 3A, to provide the initialposition estimate (Lat_(init) and Long_(init)) for the terminal.

A pseudo-range PR₁ may be derived based on the signal from SPS satellite130 x and a pseudo-range PP₁ may be derived based on the signal frombase station 120 x. The linearized equations for the terminal, with twopseudo-range measurements for one satellite and one base station, maythen be expressed as:

$\begin{matrix}{\begin{bmatrix}{\Delta \; {PR}_{1}} \\{\Delta \; {PP}_{2}} \\{\Delta \; H} \\{\Delta \; {CB}}\end{bmatrix} = {\begin{bmatrix}\frac{\partial}{\partial e} & \frac{\partial}{\partial n} & \frac{\partial}{\partial u} & 1 \\\frac{\partial}{\partial e} & \frac{\partial}{\partial n} & 0 & 1 \\0 & 0 & 1 & 0 \\0 & 0 & 0 & 1\end{bmatrix}*{\begin{bmatrix}{\Delta \; e} \\{\Delta \; n} \\{\Delta \; u} \\{\Delta \; T}\end{bmatrix}.}}} & {{Eq}\mspace{14mu} (15)}\end{matrix}$

The correction vector x may then be solved for by using equation (9) or(12) and would include two non-zero terms for Δe and Δn. The revisedposition estimate for the terminal (Lat_(rev) and Long_(rev)) may thenbe computed as shown in equation (13).

A particular coordinate (dimension) may be fixed or constrained in thederivation of the revised position estimate. For example, if signalsfrom base stations are used for updating the initial position estimate,then the vertical direction may not be observable. In this case, thealtitude coordinate in the revised position estimate may be either (1)fixed so that it is the same as that in the initial position estimate(i.e., ΔH=0) or (2) set to the predetermined level by computing thepredetermined altitude residual ΔH. Altitude may be constrained byproperly setting the observation matrix, as follows:

$\begin{matrix}{\begin{bmatrix} - \\ - \\{\Delta \; H} \\ - \end{bmatrix} = {\begin{bmatrix} - & - & - & - \\ - & - & - & - \\0 & 0 & 1 & 0 \\ - & - & - & - \end{bmatrix}*{\begin{bmatrix}{\Delta \; e} \\{\Delta \; n} \\{\Delta \; u} \\{\Delta \; T}\end{bmatrix}.}}} & {{Eq}\mspace{14mu} (16)}\end{matrix}$

As shown in equation (16), one element of the measurement vector and onerow of the observation matrix are defined so that ΔH, when applied,drives the altitude estimate to the predetermined value (where Δu can bedriven to zero or some other value). Altitude constraint can be appliedautomatically if base station measurements are used for updating. Ifsatellite and base station measurements or if only satellitemeasurements are used for updating, then altitude constraint may or maynot be applied (i.e., it is optional). Altitude constraint effectivelyprovides one of the measurements to account for one of the unknowns inthe three-dimensional positioning—height. (FIG. 3A I believe covers thiscase). FIGS. 4A through 4D are diagrams that graphically illustrate theprocess of combining the initial position estimate with SPS and/or othermeasurements. In FIG. 4A, the initial 2-D position estimate for theterminal is X_(init)=[Lat_(init), Long_(init)] and has an uncertaintydefined by an error ellipse shown by a shaded area 412 in FIG. 4A. Theerror ellipse can also be represented by a covariance measurement noisematrix, which may be expressed as:

$\begin{matrix}{{\underset{\_}{V} = \begin{bmatrix}V_{e} & V_{en} \\V_{ne} & V_{n}\end{bmatrix}},} & {{Eq}\mspace{14mu} (17)}\end{matrix}$

where

-   -   V_(e) is the variance of the error in the initial position        estimate in the east direction;    -   V_(n) is the variance of the error in the initial position        estimate in the north direction; and    -   V_(en) is the cross-correlation between the east and north        errors in the initial position estimate.        For simplicity, the cross-correlation error terms V_(en) and        V_(ne) are assumed to be zero in FIG. 4A.

In the example as depicted in FIG. 4A, where the initial positionuncertainty is represented with a covariance matrix, the initialposition estimate may be directly translated into observation equations.

$\begin{matrix}{{\begin{bmatrix}{\Delta \; {PR}_{1}} \\{\Delta \; {PP}_{1}} \\{\Delta \; E} \\{\Delta \; N} \\{\Delta \; H}\end{bmatrix} = {\begin{bmatrix}\frac{\partial}{\partial e} & \frac{\partial}{\partial n} & \frac{\partial}{\partial u} & 1 \\\frac{\partial}{\partial e} & \frac{\partial}{\partial n} & 0 & 1 \\1 & 0 & 0 & 0 \\0 & 1 & 0 & 0 \\0 & 0 & 1 & 0\end{bmatrix}*\begin{bmatrix}{\Delta \; e} \\{\Delta \; n} \\{\Delta \; u} \\{\Delta \; T}\end{bmatrix}}},} & {{Eq}\mspace{14mu} (18)}\end{matrix}$

Here, the measurement equations derived from the satellite andterrestrial positioning systems are shown as one SPS and one basestation measurement, as in Eq. (15). These equations can easily beexpanded to any number of SPS and base station measurements (e.g. as inEq. (14) and Eq. (7)) by one skilled in the art. In this example, thevalues of ΔE and ΔN are chosen to represent the estimates of initialposition with respect to the estimated terminal position around whichthe equations have been linearized. In the case where the initialposition is the estimated terminal position in a two-dimensional spacethese values can be set to 0, and 0, respectively.

In this case, the covariance matrix and weight matrices can be set torepresent the uncertainty in the initial location. For example, thecovariance matrix can be set to:

$\begin{matrix}{{\underset{\_}{V} = \begin{bmatrix}V_{PR} & 0 & 0 & 0 & 0 \\0 & V_{PP} & 0 & 0 & 0 \\0 & 0 & V_{e} & V_{en} & 0 \\0 & 0 & V_{ne} & V_{n} & 0 \\0 & 0 & 0 & 0 & V_{h}\end{bmatrix}},} & {{Eq}\mspace{14mu} (19)}\end{matrix}$

where

-   -   V_(PR) is the variance of the error for the pseudo-range        measurement PR₁ for the first satellite;    -   V_(PP) is the variance of the error for the pseudo-range        measurement PP₁ for the first base station measurement;    -   V_(e), V_(en), V_(ne), and V_(n) are set as in Equation (17)        above;

and V_(h) is set as in Equation (10) above. The weight matrix may thenbe calculated as the inverse of the covariance matrix, and the positionsolution may be calculated as in Eq. (12).

In another embodiment, the position update may be computed using maximumlikelihood methods. For example, the observation equations fromsatellite positioning systems and/or terrestrial base stationtransmitters may be used to determine the likelihood of various pointsof solution space

$\left( \begin{bmatrix}{\Delta \; e} \\{\Delta \; n} \\{\Delta \; u} \\{\Delta \; T}\end{bmatrix} \right).$

[Note: For readability I would represent solution space as a horizontalvector—transpose of the above] Additionally, the initial position,including information about east position, north position may be used torefine likelihoods at given hypothetical points representing most likelylocations (positions) of a terminal in a solution space. A covariancematrix, or more general probability density function may be used todetermine likelihoods at various locations in solution space. The heightvalue (Δu) may be fixed or constrained for determining the position ofmaximum likelihood. The solution space may also be searched for relativelikelihoods to determine an error estimate for the selected most-likelyposition.

A line-of-position (LOP) 414 can be obtained for the terminal based on aranging measurement such as an SPS measurement or a base stationmeasurement (or some other measurement). The initial position estimateX_(init) can be combined with the SPS line-of-position, as describedabove, to obtain a revised (or final) position estimate X_(rev) for theterminal. This revised position estimate would have an uncertainty (orerror) that is represented by a band along line 414. This band is notshown in FIG. 4A for simplicity. The width of the band is dependent onthe uncertainty of the underlying ranging measurement used to obtain theLOP. The banded line is bounded by the initial uncertainty, which isshaded area 412 for the error ellipse of the initial position estimate.

In FIG. 4B, the initial position estimate for the terminal is obtainedfrom a cell-ID solution, which is based on the sector of the serving BTSfor the terminal. The uncertainty of the initial position estimate maythen have a shape that approximates the pie-shaped coverage area 422 ofthe BTS (which is also referred to as a cell sector). Again, the initialposition estimate X_(init) can be combined with SPS line-of-position 424to obtain the revised position estimate X_(rev) for the terminal. Thisrevised position estimate would have an uncertainty that is representedby a band along line 424 bounded by the initial uncertainty, which isthe shaded area 422.

In FIG. 4C, the initial position estimate X_(init) for the terminal isobtained based on an enhanced cell-ID solution, which is obtained basedon the serving BTS sector 422 and the round trip delay (RTD) to thisBTS. The RTD may be obtained based on the pilot phase measurement forthe BTS. The initial position estimate X_(init) would then have anuncertainty represented by a band 432. The width of this uncertaintyband is dependent on the uncertainty (or error) in the RTD measurement.The initial position estimate X_(init) can be combined with SPSline-of-position 434 to obtain the revised position estimate X_(rev) forthe terminal.

In FIG. 4D, an accurate RTD to the BTS is obtained for the terminal.This then results in a more narrow uncertainty band 442 for the initialposition estimate X_(init). Consequently, a more accurate revisedposition estimate X_(rev) and reduced uncertainty can be obtained forthe terminal based on the initial position estimate X_(init) and SPSline-of-position 444. Note that the accurate RTD can also provide a goodΔCB measurement for an accurate reference time estimate.

In FIG. 4E, the initial position estimate X_(init) for the terminal isobtained based on an enhanced cell-ID solution. In this example, theinitial position estimate X_(init) is combined with two SPSlines-of-position 452 and 454 to obtain the revised position estimateX_(rev) for the terminal. The uncertainty in the revised positionestimate is then dependent on the uncertainties in the two SPSline-of-positions 452 and 454 and the initial position estimate.

For clarity, the examples shown in FIGS. 3A through 3C and FIGS. 4Bthrough 4D utilize the cell-ID or enhanced cell-ID technique to providethe initial position estimate for the terminal. In general, the initialposition estimate may be computed by any available positiondetermination technique. As one example, the initial position estimatemay be obtained by combining the cell-ID or enhanced cell-ID solutionsobtained for a number of base stations received by the terminal. Thismay provide a more accurate initial position estimate for the terminalsince information regarding other base stations received by the terminalis also used. As another example, the initial position estimate may beobtained by combining the modeled coverage areas for a number of basestations received by the terminal. This coverage area based positiondetermination technique is described in further detail in U.S. patentapplication Ser. No. 10/280,639, entitled “Area Based PositionDetermination for Terminals in a Wireless Network,” filed Oct. 24, 2002,(now U.S. Pat. No. 6,865,395, granted Mar. 8, 2005), assigned to theassignee of the present application and incorporated herein byreference. The initial position estimate may also be a network-basedsolution derived using A-FLT.

Various types of measurements may be used to derive lines-of-positionand consequently the revised position estimate for the terminal based onthe initial position estimate. In general, the measurements used forupdating the initial position estimate should have higher accuracy. Thatis, if a sufficient number of these measurements were available toobtain an independent position estimate for the terminal, then thatindependent position estimate would be more accurate than the initialposition estimate. Thus, if the initial position estimate is provided bythe cell-ID, enhanced cell-ID, or some other equivalent technique, thenmeasurements for base station and/or satellites may be used forupdating. This is because a network-based (A-FLT) solution derived fromonly base station measurements, a hybrid solution derived from satelliteand base station measurements, and a SPS solution derived from onlysatellite measurements are all usually more accurate than the cell-IDand enhanced cell-ID solutions. If the initial position estimate is acell-based solution, then satellite measurements may be used forupdating. In signal restrictive environments, a Local Area PositioningSystem may be used to generate an initial position estimate or be usedto update the initial position estimate derived from another source.

The number of measurements required for updating is dependent on theinitial position estimate and an update method. FIGS. 4A through 4Dillustrate how a single LOP measurement can be used to revise a 2-Dinitial position estimate. More than the minimum required number ofmeasurements may also be used for updating the initial positionestimate. For some update methods, one or more of the coordinates(time-space dimensions) (e.g., altitude, reference time) may also befixed or constrained by properly setting the observation matrix asdescribed above. In this case, fewer measurements would be needed forthe updating. For a LAPS-based update method, a single measurement canbe used.

FIG. 5 is a flow diagram of a process 200 a for providing a moreaccurate position estimate for the terminal using a partial set ofmeasurements. Process 200 a is a specific embodiment of process 200shown in FIG. 2 and is represented by FIG. 4E. Process 200 a starts offby obtaining an initial position estimate for the terminal (e.g., basedon a cell-ID solution, an enhanced cell-ID solution, or some othersolution) (step 212 a). Two measurements are also obtained for twotransmitters, each of which may be a satellite or a base station (step214 a).

The initial position estimate is then updated with the partial set ofmeasurements to obtain the revised position estimate for the terminal(step 216 a). To perform the updating, a measurement vector r is firstderived based on the initial position estimate and the measurements(step 222). Depending on the type(s) of measurements used for updating(e.g., SPS or cellular), the measurement vector may be as shown on theleft hand side in equation (7), (14), (15), or (18). An observationmatrix H is then formed for the measurements (e.g., as shown in equation(7), (14), (15) or (18)) (step 224). A matrix of weights W is nextdetermined, as described above (step 226). A correction vector x is thenobtained as shown in equation (12) (step 228). The initial positionestimate is then updated with the correction vector to obtain therevised position estimate, as shown in equation (13) (step 230). Theprocess then terminates.

Some of the position determination techniques described above may alsobe viewed as an augmentation of position (or state) domain informationwith measurement domain information for a partial set of measurements.Specifically, the augmentation described herein may be used for acell-ID based solution. Conventionally, augmentation of state domaininformation with measurement domain information requires a complete setof measurements, which greatly limits the situations where theaugmentation may be used.

FIG. 6 is a flow diagram of an embodiment of a process 600 for combiningstate domain information with measurement domain information to providea more accurate position estimate for a wireless terminal. Initially,state domain information is obtained for the terminal (step 612). Thisstate domain information may be an initial position estimate that may bederived using various techniques (e.g., cell-ID or enhanced cell-IDtechnique). Measurement domain information is also obtained for theterminal (step 614). This measurement domain information comprises apartial set of measurements that is not sufficient to derive anindependent position fix of a predetermined quality of service, but canbe combined with the state domain information.

The state domain information is then combined with the measurementdomain information to obtain a position estimate for the terminal havingan accuracy at least as good as that of the state domain (step 616).

FIG. 7 is a block diagram of an embodiment of a receiver unit 700, whichmay be a component of a wireless terminal. Receiver unit 700 may bedesigned with the capability to process signals from multiple positiondetermination systems such as the SPS and wireless communication system.In the embodiment shown in FIG. 7, receiver unit 700 includes an antenna710, a terrestrial receiver 712 a, an SPS receiver 712 b, a processingunit 716, a memory unit 718, and a controller 720.

Antenna 710 receives signals from a number of transmitters (which may beany combination of SPS satellites and/or base stations) and provides thereceived signal to terrestrial and SPS receivers 712 a and 712 b.Terrestrial receiver 712 a includes front-end circuitry (e.g., radiofrequency (RF) circuitry and/or other processing circuitry) thatprocesses the signals transmitted from base stations to obtaininformation used for position determination. For example, terrestrialreceiver 712 a may measure the phase of the pilot in the forward linksignal received from each base station to obtain timing information(e.g., time or arrival). This timing information may thereafter be usedto derive a pseudo-range to the base station.

Terrestrial receiver 712 a may implement a rake receiver that is capableof concurrently processing multiple signal instances (or multipathcomponents) in the received signal. The rake receiver includes a numberof demodulation elements (often known as fingers), each of which may beassigned to process and track a particular multipath component. Eventhough multiple fingers may be assigned to process multiple multipathcomponents for a given base station, only one pseudo-range obtained forone multipath component (e.g., the earliest arriving multipathcomponent, or the strongest multipath component) is typically used forposition determination. Alternatively, a timing (or ranging)relationship between different fingers may be established andmaintained. In this way, it is possible to use different multipathcomponents for a given base station for position determination dependingon the fading and multipath effects.

SPS receiver unit 712 b includes front-end circuitry that processessignals transmitted from SPS satellites to obtain information used forposition determination. The processing by receivers 712 a and 712 b toextract the pertinent information from the SPS and terrestrial signalsare known in the art and not described in detail herein. In oneembodiment, SPS signal processing may be performed by terrestrialreceiver unit 712 a. Receivers 712 a and 712 b provide to processingunit 716 various types of information such as, for example, timinginformation, signal characteristics, the identities and locations of thetransmitters whose signals are received, and so on.

Processing unit 716 may obtain an initial position estimate for receiverunit 700 whenever requested. Processing unit 716 may also determine apseudo-range residual for each base station and satellite to be used toupdate the initial position estimate, as described above. Processingunit 716 may thereafter update the initial position estimate based onthe pseudo-range residuals to obtain a revised position estimate for thereceiver unit.

Memory unit 718 stores various data used for determining position. Forexample, memory unit 718 may store information for the locations of theSPS satellites (which may be derived from the Almanac and/or Ephemeristransmitted by the satellites or provided by the terrestrial source(e.g., wireless network)), the locations of the base stations (which maybe provided via signaling), and the pseudo-range residuals. Memory unit718 may also store program codes and data for processing unit 716.

Controller 720 may direct the operation of processing unit 716. Forexample, controller 720 may select the particular types of solution tobe computed (e.g., SPS-based, network-based, hybrid, cell-based, LAPS,safety-net, and other combined solutions), the particular algorithm tobe used (if more than one is available), and so on.

Although not shown in FIG. 7, receiver unit 700 may communicate with alocation server 140 (see FIG. 1), which may assist in determining theterminal's position estimate. The location server may perform thecomputations to derive the position estimate, or may provide certaininformation used to (1) acquire satellite and/or base stationmeasurements (e.g., acquisition assistance, timing assistance,information related to the location of the SPS satellites and/or basestations, and so on) and/or (2) determine the revised position estimate.For the embodiments whereby the location server performs positiondetermination, the underlying measurements from various positioningsystems and the initial position estimate are communicated to thelocation server (e.g., via wireless and/or wireline links). An exampleof such a location server is described in U.S. Pat. No. 6,208,290, whichis incorporated herein by reference.

The method and apparatus described herein may be used in conjunctionwith various wireless communication systems and networks. For example,the disclosed method and apparatus may be used for CDMA, time divisionmultiple access (TDMA), frequency division multiple access (FDMA), andother wireless communication systems. These systems may implement one ormore applicable standards. For example, the CDMA systems may implementIS-95, cdma2000, IS-856, W-CDMA, and so on. The TDMA systems mayimplement GSM, GPRS and so on. These various standards are known in theart and incorporated herein by reference. The other wirelesscommunication systems include non-cellular wireless systems such as, forexample, IEEE 802.11 systems, Bluetooth systems, and wireless local areanetworks (WLANs).

The method and apparatus described herein may be used with varioussatellite positioning systems (SPS), such as the United States GlobalPositioning System (GPS), the Russian Glonass system, and the EuropeanGalileo system. Furthermore, the disclosed method and apparatus may beused with positioning determination systems that utilize pseudolites ora combination of satellites and pseudolites. Pseudolites areground-based transmitters that broadcast a PN code or other ranging code(similar to a GPS or CDMA cellular signal) modulated on an L-band (orother frequency) carrier signal, which may be synchronized with GPStime. Each such transmitter may be assigned a unique PN code so as topermit identification by a remote receiver. Pseudolites are useful insituations where GPS signals from an orbiting satellite might beunavailable, such as in tunnels, mines, buildings, urban canyons orother enclosed areas. Another implementation of pseudolites is known asradio-beacons. The term “satellite”, as used herein, is intended toinclude pseudolites, equivalents of pseudolites, and possibly others.The term “SPS signals”, as used herein, is intended to include SPS-likesignals from pseudolites or equivalents of pseudolites. The term “basestation”, as used herein, is intended to include cellular, wireless,LAN, WAN, LAPS, Bluetooth, 802.11 access points and other terrestrialsources of signals.

The method and apparatus described herein may be implemented by variousmeans, such as in hardware, software, or a combination thereof. For ahardware implementation, the method and apparatus may be implementedwithin one or more application specific integrated circuits (ASICs),digital signal processors (DSPs), digital signal processing devices(DSPDs), programmable logic devices (PLDs), field programmable gatearrays (FPGAs), processors, controllers, micro-controllers,microprocessors, other electronic units designed to perform thefunctions described herein, or a combination thereof.

For a software implementation, the disclosed method may be implementedwith modules (e.g., procedures, functions, and so on) that perform thefunctions described herein. The software codes may be stored in a memoryunit (e.g., memory 718 in FIG. 7) and executed by a processor (e.g.,processing unit 716 or controller 720). The memory unit may beimplemented within the processor or external to the processor, in whichcase it can be communicatively coupled to the processor via variousmeans as is known in the art.

The previous description of the disclosed embodiments is provided toenable any person skilled in the art to make or use the presentinvention. Various modifications to these embodiments will be readilyapparent to those skilled in the art, and the generic principles definedherein may be applied to other embodiments without departing from thespirit or scope of the invention. Thus, the present invention is notintended to be limited to the embodiments shown herein but is to beaccorded the widest scope consistent with the principles and novelfeatures disclosed herein.

What is claimed is:
 1. (canceled)
 2. A method of determining a positionestimate for a wireless terminal, comprising: obtaining an initialposition estimate for the terminal using a first position determinationscheme based at least in part on one or more signals from a firstwireless communication system; obtaining a set of one or moremeasurements using at least one additional position determination schemebased on at least one signal from at least one additional wirelesscommunication system that is distinct from the first wirelesscommunication system; and updating the initial position estimate basedon the set of one or more measurements.
 3. The method of claim 2,wherein the first wireless communication system corresponds to acellular communication system or a Local Area Positioning System (LAPS).4. The method of claim 3, wherein the first position determinationscheme corresponds to a WiFi-based position determination scheme, aBluetooth-based position determination scheme, an Ultra-Wide Bandwidth(UWB) position determination scheme, cell-ID position determinationscheme, an enhanced cell-ID position determination scheme, a LAPS-basedposition determination scheme, an Advanced Forward Link Trilateration(A-FLT) position determination scheme, an Uplink Time of Arrival (U-TOA)or Uplink Time Difference of Arrival (U-TDOA) position determinationscheme, an Enhanced Observed Time Difference (E-OTD) positiondetermination scheme, and/or an Observed Time Difference of Arrival(OTDOA) position determination scheme.
 5. The method of claim 3, whereinthe first wireless communication system corresponds to the LAPS.
 6. Themethod of claim 4, wherein the LAPS is a Wireless Local Area Network(WLAN) or WiFi communication system.
 7. The method of claim 6, whereinthe at least one additional wireless communication system includes aSatellite Positioning System (SPS).
 8. The method of claim 7, whereinthe set of one or more measurements includes one or more pseudorangemeasurements to one or more satellite or pseudolite transmitters.
 9. Themethod of claim 6, wherein the at least one additional wirelesscommunication system includes the cellular communication system.
 10. Themethod of claim 9, wherein the set of one or more measurements includesat least one measurement of an Observed Time Difference of Arrival(OTDOA) for the at least one signal.
 11. The method of claim 3, whereinthe first wireless communication system corresponds to the cellularcommunication system.
 12. The method of claim 11, wherein the initialposition estimate is obtained based on the one or more signals of thecellular communication system using a cell-ID position determinationscheme or an enhanced cell-ID position determination scheme.
 13. Themethod of claim 12, wherein the at least one additional wirelesscommunication system includes the LAPS.
 14. The method of claim 13,wherein the LAPS is a Wireless Local Area Network (WLAN) or WiFicommunication system.
 15. The method of claim 14, wherein the set of oneor more measurements includes a set of one or more WiFi-based orWLAN-based round trip time (RTT) measurements measured using the atleast one signal from the WLAN or WiFi communication system.
 16. Themethod of claim 2, wherein the at least one additional positiondetermination scheme includes a Satellite Positioning System (SPS)position determination scheme, a Local Area Positioning System (LAPS)position determination scheme, an Advanced Forward Link Trilateration(A-FLT) position determination scheme, a hybrid SPS+A-FLT positiondetermination scheme, an Uplink Time of Arrival (U-TOA) or Uplink TimeDifference of Arrival (U-TDOA) position determination scheme, anEnhanced Observed Time Difference (E-OTD) position determination scheme,and/or an Observed Time Difference of Arrival (OTDOA) positiondetermination scheme.
 17. The method of claim 2, wherein the at leastone additional wireless communication system includes a cellularcommunication system, a Local Area Positioning System (LAPS) and/or asatellite communication system.
 18. The method of claim 17, wherein theset of one or more measurements includes a LAPS-based measurement and acellular-based measurement.
 19. The method of claim 18, wherein thecellular-based measurement is a measurement of an Observed TimeDifference of Arrival (OTDOA).
 20. The method of claim 19, wherein theLAPS is a Wireless Local Area Network (WLAN) or WiFi communicationsystem.
 21. The method of claim 20, wherein the LAPS-based measurementis a WiFi-based or WLAN-based round trip time (RTT) measurement.
 22. Themethod of claim 2, wherein the initial position estimate is a bestavailable position estimate that is independently derived by via anyavailable position determining scheme, and wherein the set of one ormore measurements used to update the initial position estimate isobtained in conjunction with an earlier attempt to independently derivethe position estimate of the wireless terminal by the at least oneadditional position determination scheme.
 23. The method of claim 2,wherein the set of one or more measurements is a partial set of one ormore measurements that is insufficient to independently derive theposition estimate for the terminal with a threshold level of accuracyusing the at least one additional determination scheme.
 24. The methodof claim 23, wherein the threshold level of accuracy is establishedbased on an application-specific accuracy requirement for the positionestimate.
 25. The method of claim 2, wherein the set of one or moremeasurements is a complete set of one or more measurements that issufficient to independently derive the position estimate for theterminal with a threshold level of accuracy using the at least oneadditional determination scheme.
 26. The method of claim 25, wherein thethreshold level of accuracy is established based on anapplication-specific accuracy requirement for the position estimate. 27.The method of claim 2, wherein the at least one additional wirelesscommunication system includes a single wireless communication system.28. The method of claim 2, wherein the at least one additional wirelesscommunication system includes multiple additional wireless communicationsystems.
 29. The method of claim 2, wherein the at least one additionalposition determination scheme is associated with higher accuracyposition estimates as compared to the first position determinationscheme.
 30. The method of claim 2, wherein the updating includesweighting the set of one or more measurements based on an expecteddegree of error, and wherein a degree to which the initial positionestimate is adjusted during the updating is based on the weighting. 31.The method of claim 2, wherein the updating adjusts a latitudecomponent, a longitude component and/or an altitude component associatedwith the initial position estimate based on the set of one or moremeasurements.
 32. The method of claim 2, wherein the set of one or moremeasurements includes at least one ranging measurement to a transmitter.33. The method of claim 2, wherein the set of one or more measurementsincludes at least two ranging measurements to different transmitters.34. The method of claim 33, wherein the at least two rangingmeasurements include at least one terrestrial ranging measurement to atleast one terrestrial transmitter and at least one satellite rangingmeasurement to at least one satellite or pseudolite transmitter.
 35. Anapparatus configured to determine a position estimate for a wirelessterminal, comprising: means for obtaining an initial position estimatefor the terminal using a first position determination scheme based atleast in part on one or more signals from a first wireless communicationsystem; means for obtaining a set of one or more measurements using atleast one additional position determination scheme based on at least onesignal from at least one additional wireless communication system thatis distinct from the first wireless communication system; and means forupdating the initial position estimate based on the set of one or moremeasurements.
 36. The apparatus of claim 35, wherein the first wirelesscommunication system corresponds to a cellular communication system, andwherein the at least one additional wireless communication systemincludes a Local Area Positioning System (LAPS) and/or a satellitepositioning system (SPS).
 37. The apparatus of claim 35, wherein thefirst wireless communication system corresponds to a Local AreaPositioning System (LAPS), and wherein the at least one additionalwireless communication system includes a cellular communication systemand/or a satellite positioning system (SPS).
 38. An apparatus configuredto determine a position estimate for a wireless terminal, comprising: aprocessor configured to obtain an initial position estimate for theterminal using a first position determination scheme based at least inpart on one or more signals from a first wireless communication system,to obtain a set of one or more measurements using at least oneadditional position determination scheme based on at least one signalfrom at least one additional wireless communication system that isdistinct from the first wireless communication system and to update theinitial position estimate based on the set of one or more measurements.39. The apparatus of claim 38, wherein the first wireless communicationsystem corresponds to a cellular communication system, and wherein theat least one additional wireless communication system includes a LocalArea Positioning System (LAPS) and/or a satellite positioning system(SPS).
 40. The apparatus of claim 38, wherein the first wirelesscommunication system corresponds to a Local Area Positioning System(LAPS), and wherein the at least one additional wireless communicationsystem includes a cellular communication system and/or a satellitepositioning system (SPS).
 41. A non-transitory computer-readable mediumcontaining instructions stored thereon, which, when executed by anapparatus configured to determine a position estimate for a wirelessterminal, cause the apparatus to perform operations, the instructionscomprising: at least one instruction configured to cause the apparatusto obtain an initial position estimate for the terminal using a firstposition determination scheme based at least in part on one or moresignals from a first wireless communication system; at least oneinstruction configured to cause the apparatus to obtain a set of one ormore measurements using at least one additional position determinationscheme based on at least one signal from at least one additionalwireless communication system that is distinct from the first wirelesscommunication system; and at least one instruction configured to causethe apparatus to update the initial position estimate based on the setof one or more measurements.
 42. The non-transitory computer-readablemedium of claim 41, wherein the first wireless communication systemcorresponds to a cellular communication system, and wherein the at leastone additional wireless communication system includes a Local AreaPositioning System (LAPS) and/or a satellite positioning system (SPS).43. The non-transitory computer-readable medium of claim 41, wherein thefirst wireless communication system corresponds to a Local AreaPositioning System (LAPS), and wherein the at least one additionalwireless communication system includes a cellular communication systemand/or a satellite positioning system (SPS).